The present invention relates generally to electrophotographic image forming devices, and in particular to preconditioning media sheets to reduce the required transfer voltage.
Electrophotographic image forming devices, such as laser printers, are well known in the art and widely deployed. Color electrophotographic image forming devices form a plurality of latent electrostatic images, develop each color plane image with toner particles, and ultimately transfer the color plane images to a media sheet and then fuse them to the media sheet using heat and pressure. Color electrophotographic image forming devices may be divided into to types by considering how toner is transferred to media sheets. In a direct to media (DTM) type image forming device, the developed toner image of each color plane is successively transferred directly to the media sheet. In an intermediate transfer mechanism (ITM) type image forming device, the developed toner image of each color plane is successively transferred to an intermediate mechanism, such as a belt, and then the full-color image is transferred to a media sheet.
One known problem that particularly affects DTM type image forming devices is that resistive media sheets become charged as they pass successively through high-voltage image transfer stations. Accordingly, to maintain high image transfer quality, the transfer voltage at downstream image transfer stations must be increased, to offset the effects of the media sheet accumulating ever-greater charge as it progresses through upstream image transfer stations. While this technique works well to preserve image transfer quality, there are practical limits to the voltage levels that downstream image transfer stations an employ. First, very high transfer voltages may require more expensive high-voltage power supplies. Second, at very high transfer voltages, air may ionize in the region surrounding downstream image transfer stations, a phenomenon known as Paschen breakdown. In Paschen breakdown, toner particles reverse polarity and their placement becomes unpredictable—a phenomenon known as backtransfer. Backtransfer detrimentally impacts image quality. Additionally, in some case monochrome DTM type and ITM type image forming devices may require very high transfer voltages, such as when transferring images to very highly resistive media.